In recent years, from viewpoints of cost reduction, ease of molding, improved workability, productivity, improved handling and processing, energy saving, space saving, increased safety, and stricter environmental protection, it has so far been investigated in various fields to use energy-ray curing resins which are characterized by energy-ray curing such as UV curing. However, insufficient energy-ray curing capability has been given as a factor which inhibits their use.
Energy-ray curing resins such as UV-curing resins is characterized by that only a part irradiated with a certain amount or more of an energy ray is cured, and energy rays such as UV rays are attenuated in the course of transmitting through the resin, so that energy-ray curing is influenced to a large extent by a curing capacity of the resin itself and the intensity, irradiation time and attenuation characteristic of the energy ray.
In order to expand the utilization of this technique and apply it to various fields in the future, higher curing capacities are required in many cases, and methods which have so far been carried out in order to improve an energy-ray curing capacity include an improved performance of a photoinitiator, a rise in the intensity of an energy ray irradiated, an extension of the irradiating time, and a change in the kind of energy rays.
However, when employing the methods described above, time and cost required for developing an initiator and expensive resin compositions have been considered as problems concerning the resin composition. Also, problems such as the requirement of larger apparatuses, increased energy consumption, higher running costs, reduced productivity, special requirements on ray sources, high costs of the apparatuses and the facilities and reduced safety have been involved with respect to energy ray irradiation apparatuses and facilities. Thus, it is difficult to use and apply the above-mentioned methods unless problems such as lost advantages of using energy rays and increased total costs are solved.
For example, a cured film thickness of an ordinary energy-ray curing resin is several μm to several mm on a surface which is effectively reached by an energy ray, and if a transmitting distance is extended, an effective amount of the energy ray does not reach, and portions beyond a certain depth are not cured. In order to improve the curing capacity, considered are, excluding the degree of the effects thereof, a change in the resin composition, an increase in the intensity of an energy ray irradiated, and a change in the ray source. In this case, however, problems similar to those discussed above also arise.
Accordingly, applicable fields of energy-ray curing have so far been limited to areas such as photoresists, coatings, paints, adhesives, varnishes and the like.
Representative examples for improving the energy-ray curing capacity include a high UV-curing resin (active energy-ray curing composition disclosed in Japanese Patent Application Laid-Open No. 8-283388, filed in the name of Mitsubishi Rayon Co., Ltd.) and UV-heat combined curing type resins (Optomer KS series by Asahi Denka Ind. Co., Ltd.; Redicure by Hitachi Kasei Ind. Co., Ltd.; UE resins by Toyo Boseki Co., Ltd.; and Japanese Patent Publication (Kokoku) No. 61-38023).
However, conventional high curability energy-ray curing resins represented by a high UV-curing resin have so far been dependent on the development of novel photopolymerizable initiators which are effective for energy-ray curing or, though examples thereof are fewer than the above, on the development of novel photopolymerizable oligomers. The problems described above are also involved therein, and it is hardly recognized possible to readily obtain compositions suitable to particular uses. Also, the UV-heat combined curing type resins have broader curing conditions. On the other hand, they have the same problems as those associated with the high-curability energy-ray curing resins. Further, the requirement of a heating process leads to the necessity of heating devices and related facilities, so that the advantages of energy-ray curing techniques are reduced with respect to overall requirements or apparatuses and facilities.